ENH: avoid clearStorage for persistent PstreamBuffers

- added persistent buffer for zoneDistribute
- additional handling for clearing buffers

src/functionObjects/field/DMD/DMDModels/derived/STDMD/STDMD.C

@@ -226,7 +226,9 @@ reducedKoopmanOperator()
         // Tests revealed that the distribution of "Q" does not affect
         // the final outcome of TSQR decomposition up to sign
 
+        // Don't clear storage on persistent buffer
         PstreamBuffers pBufs(Pstream::commsTypes::nonBlocking);
+        pBufs.allowClearRecv(false);
 
         const label myProcNo = Pstream::myProcNo();
         const label procNoInSubset = myProcNo % nAgglomerationProcs_;